I can only give you some stray tidbits of general information. You probably need to spend some time googling for schematics and other info. Look for a big-rig mechanic's forum also, in case there is one.
And you could try the SchoolBusFleetMagazine forum -- they are "real" school bus people, but they have helped me with information on several occasions. Might be a big-rig mechanic among them. I suggest you invest some time making sure your inquiry there is very well written; thoroughly thought out, good syntax, correct grammar and spelling, so they don't blow you off as the kid they had to throw off their own bus yesterday. Use their "Garage" forum section.
But the best two sources would probably be the wrecking yard where you bought the parts, and a Freightliner dealer.
The wrecking yard ought to let you crawl under a similar truck with notepad and camera. You should also ask if they have a man who can explain it to you, but they may not have.
At the dealer, you should be able to just slide under a (nice clean!) new truck, again with notepad and camera. And they certainly will have knowledgeable people there. Now, since I don't know you, I'm going to tell you something that may or may not apply to you: When you go to the dealer, you should appear VERY mature and professional. If you and the dealership manager are standing next to each other, it should not be possible to guess which is which. I say this because big rig dealerships suffer under a steady flow of scraggly-haired cigarette-smoking buffoons who drive trucks, and you need to separate yourself from that bunch. You should appear to be a well-to-do grocery store manager who just might need to buy a small fleet of new Freightliners in another five years. Of course, I'm laying it on a bit heavy, but hopefully this makes sense.
Exactly how you proceed when you arrive, depends on the dealership. A large Freightliner dealer may have trucks on display on the front lawn that you can crawl all over on a Sunday morning without anybody ever knowing you are even there. At the other extreme, at a small shop somebody may shout "Hi, may I help you?" the moment you step out of your car.
Carry a clipboard with notepad, and a camera, and act with a purpose. No hands in pockets!
If you see a mechanic out in the yard, chat him up. If not, go in to the Service or Parts counter and chat that guy up, and show him big glossy photos of your bus project. Did I mention photos of your bus? Some roof raising photos will go a long way to establishing your credentials.
All right. Unless someone more knowledgable gives you detailed instructions for two valves, use one leveling valve. It goes on a crossmember near the axle. The valve has a lever, and the lever attaches to a rod that attaches to the axle. The length of this rod may be adjustable. There is probably a rubber bushing at each end, to allow movement. The valve should be in the middle of its travel when the suspension is in the middle of its travel. For correct suspension height, you will probably need Freightliner information. Then, you find the middle of the valve travel by lining up two small holes -- it's intended to stick a pin in it to hold it there while you adjust the rod.
The valve has a time delay feature so that no air moves when the suspension jumps up and down while driving. But when the valve moves to a new position for a longer time, such as when you load more cargo in the bus, then the valve changes the amount of air in the bags until the valve is level again. More weight = more air. Less weight = a hissing sound as air is let out.
(A (single axle) two-valve system would have one valve on each side, each controlling the bag on that side. This minimizes leaning in curves, methinks.)
The air suspension does not regulate your ride comfort. It controls the distance between the axle and the frame (on smooth ground). The leveling valve keeps this constant, no matter what. Ride comfort will change with the weight of bus and cargo, and the suspension does not know nor care. BUT... a lump of air is a much more compliant and sensitive spring than a slab of steel, and this is where you get your improved comfort. And the vehicle stays level in the bargain.
You will want to be certain that you install any additional valves that belong to the system, such as any that might regulate flow from side to side. I don't know what there might be, but you need to find out.
There is certain to be some adjustment in the suspension for aligning the axle in the chassis. It may be by shims, or by some sort of eccentrics, or both. Of course, you still need to install the brackets as accurately as possible, and only fine tune with the adjustments. My guess is, you can get very close by measuring from existing holes in the frame nearby, such as the old spring mounts.
As for crossmembers, you probably want to duplicate the Freightliner arrangement.
You shouldn't need more than one tank, except that two tanks in series is a good way to keep moisture out of the valves and bags.
If you can make friends with a big rig mechanic, you might consider hiring him for a weekend.
I don't know what else to suggest for now.